It is common for a system to have an integrated circuit that uses one or more external resistors to set a threshold, trip point, or other parameter for the system. This provides users with a great deal of flexibility when using the same integrated circuit. For example, each user could set the trip point of a temperature sensor by selecting the appropriate value(s) for the external resistor(s). The alternative would be to manufacture hundreds of different integrated circuits for hundreds of different trip points, which is not practical.
There are two common ways of setting a parameter value based on an external resistor. One method uses an external voltage source and a resistive divider. By choosing the appropriate resistor values, one can establish any desired threshold voltage. The other method has the integrated circuit using a current source to provide a fixed current that drives an external resistor. By choosing an appropriate resistor value, the user can establish any desired threshold voltage.
A disadvantage of these two approaches is that the external resistor(s) and the voltage or current source introduce tolerances on the threshold or other parameter. In other words, variations in these components can cause variations in the threshold or other parameter output by the integrated circuit.